Constituent Monomer Oligomer

[g kg–1 wood]

Xylose 18.0 100.6

Mannose 0.0 1.6

Glucose 2.5 4.9

Arabinose 3.8 0.0

Galactose 2.9 4.0

Rhamnose 2.0 1.4

1.5 2.0 2.5 3.0 3.5 4.0

0.00

0.05

0.10

0.15 Mw = 880 g/mol

Mn = 300 g/mol

weight fraction

Log Molar Mass

Fig. 4.103 Size-exclusion chromatography of water prehydrolyzate

from beech wood after a treatment of 1 hour at 170 °C

at a liquor-to-solid ratio of 10:1 (according to [44]). (PSS MCX

1000 columns; 0.5 M NaOH; flow rate 1 mL min–1).

The calculated molecular weights were significantly lower as reported from the

literature [42], and comprised a weight-average degree of polymerization (DP) of

about 7. These values were in agreement with those determined using high-performance

anion-exchange chromatography (HP-AEC) with pulsed amperometric

detection and coupled to mass spectrometry (Fig. 4.104).

340 4 Chemical Pulping Processes

0 10 20 30 40

0.0

0.5

1.0

1.5

C5-4OMeGlc p A neutral C5-sugars

EIC EIC PAD

DP6

A*

DP5

A*

DP4

A*

DP3

A*

DP2

A*

DP7 DP8 DP9

DP6

DP5

DP3 DP4

DP2

DP1

Intensity [counts]

Retention time [min]

Fig. 4.104 High-performance anion-exchange

chromatography HP-AEC coupled to pulsed

amperometric detection (PAD) and to mass

spectrometry of water prehydrolyzate from

beech wood after a treatment of 1 h at 170 °C

at a liquor-to-solid ratio of 10:1 (according to

[44]). (HP-AEC-PAD: Dionex CarboPac PA100,

0.15 M NaOH/0.5 M NaAc, flow rate

0.5 mL min–1. MS-detection: Esquire 3000plus:

ESI, negative mode, flow rate 0.5 mL min–1,

extracted ion chromatography, EIC). A*: 4- O methyl-

b-d-glucuronic acid.

The dissolved hemicellulose fragments undergo further acid-catalyzed hydrolysis

to monosaccharides only after applying rather severe conditions. Even after

200 min at 170 °C, the proportion of xylose monomers in the prehydrolyzate

amounts to only 50%. At the same time, the decomposition reactions – as shown

schematically in Scheme 4.30 and Eq. (148) – begin significantly to diminish the

overall xylose yield. The yield of monomer xylose can be substantially increased

only by either applying prehydrolysis with dilute solutions of sulfuric acid or by

raising the temperature beyond 170 °C in the case of water prehydrolysis [1].

Oligomeric arabinose structures are rapidly cleaved to monomeric sugars, however.

The conversion of galactose and glucose oligomers to the monomers proceeds

with intermediate reaction rates.

The deacetylation of xylan governs the efficiency of the prehydrolysis reactions.

Surprisingly, the molar ratio of acetic acid in solution to xylose removed from the

residue increases from 0.4:1.0 to a saturation value of slightly above 0.7:1.0 during

the first 100 min of retention time at 170 °C. These results (see Fig. 4.105) indicate

that deacetylation in the early stages of prehydrolysis occurs at a somewhat slower

rate than xylan removal, whereas the two removal rates are almost equal during

the final course of water prehydrolysis.

Figure 4.105 also displays the molar ratio of methanol released to the xylose

removed. Again, in the initial phase of prehydrolyis the xylan removal rate seems

to be ahead of the splitting of the methoxyl group from the xylan molecule.

4.2 Kraft Pulping Processes 341

0 50 100 150 200

0.00

0.05

0.10

0.4

0.5

0.6

0.7

0.8

Acetic Acid Methanol

Molar Ratio [n: Xylose]

Time at 170.C, min

Fig. 4.105 Molar ratio of acetic acid and methanol in solution

to xylose removed from the residue as a function of reaction

time during water prehydrolysis of beech wood at 170 °C

(according to [39]). Liquor-to-solid ratio = 10:1.

Furthermore, lignin–carbohydrate bonds and some inter-unit lignin bonds,

mainly derived from the benzyl alkyl ether type, may be cleaved during water prehydrolysis.

Consequently, lignin compounds are also removed from the solid

wood residue. As xylan removal progresses, the amount of lignin removed passes

through a maximum and than greatly decreases [1]. This apparent decrease has

been attributed to a redeposition of carbohydrate degradation products on the

wood residue that have been determined as lignin, as they are also insoluble in

72% sulfuric acid. Recently, lignin measurements (Klason lignin and acid-soluble

lignin) on beech wood revealed that water prehydrolysis contributes to a substantial

degradation of lignin compounds. At 60% xylan removal, water prehydrolysis

at 170 °C and a liquor-to-solid ratio of 10:1 removes about 26% of lignin. It was

reported that the so-called Hibbert ketones (e.g., vanilloyl methyl ketone and

guaiacyl acetone, coniferylaldehyde and p -coumaraledehyde) which obviously originate

from lignin, were present in the water prehydrolyzate from hemlock [45].

Coniferylaldehyde was found to be the major product of acid-catalyzed hydrolysis

of guaicylglycerol-b-aryl ether at pH 5 and 175 °C [46]. Parallel to the decrease in

the lignin content, the amount of material extractable by means of organic solvents

[e.g., dichloromethane (DCM) or ethanol-benzene]increases [47]. This may

be explained by the fact that, under prehydrolysis conditions, acid hydrolysis

causes depolymerization of lignin and renders parts of the lignin soluble in water

or organic solvents. The amount of DCM extractives in beech wood increases

from 0.3% in the untreated wood to 2.0% after a 30-min vapor phase prehydrolysis

treatment at 170 °C [48].

342 4 Chemical Pulping Processes

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